Semiconducting materials and devices made therefrom



April 14, 1959 J. H. WERNICK 2,882,467

ssmcoupucwmc MATERIALS AND DEVICES MADE THEREFROM Filed May 10, 1957 VlNl ENTOR J. H. WERN/CK IISEMICONDUCTING MATERIALS AND DEVICES MADETHEREFROM lack H. Wernick, Morristown, N.J., assignor to Bell TelephoneLaboratories, Incorporated, New York, N.Y., a corporation of New YorkApplication May 10, 1957, Serial No. 658,431

15 Claims. (Cl. 317-237) This invention relates to ternarysemiconductive compounds and to semiconductive devices containing suchcompounds.

I In accordance with this invention there has been discovered a newseries of semiconducting compounds of the general composition AgXSe inwhich X is antimony, bismuth or arsenic. These new materials haveintrinsic energy gaps between 0.6 and 1.0 electron volts, a range makingthem useful in the construction of common semiconductor devices such,for example, as rectifiers and transistors and also in photo devicessuch as infrared detectors. All of these materials in addition to beingin-' trinsic semiconductors evidence extrinsic semiconductive propertiesso that they are useful both in point-type and in junction-type devices.

2 is n-type, ready conduction occurs with electrode 1 biased positivewith respect to base 3. Where the material of block 2 is p-type readyconduction occurs with electrode 1 biased negative with respect to base3. I

The device of Fig. 2 is a junction-type diode consisting of electrode 11making ohmic connection 12 with surface 13 of block 14 which may, forexample, be AgSbSe and which block contains p-n junction 15 betweenregion 16 which is of one conductivity type and region 17 of theopposite conductivity type. Semiconductor block 14 makes ohmic contactwith electrode 18 by means, for

example, of a solder joint at 19. As will be discussed, where block 14is silver antimony selenide which is of p-type conductivity as made,region 17 may constitute the unconverted material and, therefore, be ofp-type conductivity, while region 16 of n-type conductivity may beproduced, for example, by doping with a significant impurity such asiodine from group VII of the periodic table according to Mendelyeev.

In the description of the device of Fig. 2 as in the description of thedevice of Fig. 1, it is not considered to be These new compounds arediscussed herein in terms of their electrical and physical propertiesand their use in two typical semiconductor transducing devices; onepointtype and one junction-type. Since none of the materials which isthe subject of this invention is known to occur in nature, a method bywhich each of them has been synthesized is described.

I The invention may be more easily understood by reference to thefollowing figures in which:

Fig. 1 is a schematic front elevational view in section of a point-typediode utilizing one of the compounds herein;

. Fig. 2 is a schematic front elevational view in section of ajunction-type diode utilizing one of the compounds herein; and

Fig. 3 is a schematic cross-sectional view of apparatus used in thepreparation of each of the compounds of this invention.

Referring again to Fig. l, point-electrode 1 makes rectifying contactwith semiconductor block 2 which may contain any one or more of thecompounds of this invention, silver antimony selenide, silver bismuthselenide or silver arsenic selenide, so modified by the incorporation ofone or more significant impurities or other means as to exhibitextrinsic conductivity. Semiconductor block 2 makes ohmic contact withbase 3 which may be made, for example, of copper. As is well known tothose skilled in the art, such ohmic connection may be made, forexample, by use of a solder containing a material having an excess ofelectrons where the material of semiconductor block 2 is n-type and adeficiency of electrons where the material of; serniconductor block 2 isp-type. Methods of making satisfactory point contact are well known andare not. discussed. For suitable materials for the construction of apoint-type electrode such as electrode 1 and for suitable methods ofpointing such electrodes and bringing them to bear on the surface ofblock 2, attention is diwithin the scope of this description to setforth contacting means and other design criteria well known to thosefamiliar with the fabrication of semiconductive devices.

Fig. 3 depicts one type of appartus found suitable for the preparationof each of the three semiconductive compounds herein. Reference will bemade to this figure in the examples relating to the actual preparationof these compounds. The apparatus of thisfigure consists of a resistancewire furnace '25 containing three individual windings 26, 27 and 28 asindicated schematically, these windings comprising turns of platinum-2Opercent rhodium resistance wire.. In operation,.an electrical potentialis applied across terminals 29 and 30 and also across terminals 31 and32 by means not shown. The amount of current passing through resistancewinding 27 is con-' trolled by means of an autotransformer 33, while theamount of current supplied to windings 26 and 28 is controlled byautotransformer 34, so that the temperature of the furnace withinwinding 27 may be controlled independently of the temperature in thefurnace within windings 26 and 28. Switch 35 makes possible the shuntingof winding 28 while permitting current to pass through winding 26. Thefunctions served by autotransformers 33 and 34 and switch 35 areexplained in conjunction with the general description of the method ofsynthesis.

Within furnace 25 there is contained sealed container 36 which may bemade of silica and may, for example, be of an inside diameter of theorder of 19 millimeters within which there is sealed a second silicacrucible 37 containing the component materials 38 used in the synthesisof a compound of this invention. Coating 39 on the inner surface ofcrucible 37 may be of a material such as carbon and has the effect ofreducing adhesion between surface 39 and the final compound. Innercrucible 37 is closed at its upper end with graphite cap 40 having hole41 so as to prevent possible boiling over into container 36 and tominimize heating of charge during sealing ofr of container 36. In thesynthesis of the materials herein thermal losses are reduced andtemperature control gained by use of insulation layers 43 and 44 whichmay, for example be Sil-o-cell refractory.

The following is a general outline of a method of preparation used-insynthesis of the compounds of this invention. Reference will be had tothis general outline in Examples 1 through 3 each of which sets forththe specific starting materials and conditions of processing utilized inthe preparation of a compound herein.

In the preparation of the selenides of this invention, it

was foundnecessary to coat the inner surface of the inner Patented A pr.1959 p the: materials: therein contained. It was found that asuitable-coatingwas-pwducedByeXposureofthe'crucibleto a mixture of fourparts of nitrogen and one part of methane for a period of 15" minutes, aflow rate of approxir'n'ately 2 i)"cubic-c'entiineters= per minute-withacru-- cible at-a-temperature-of about 1000" C'. After coatihg the chargewas pl'a'cedih crucible37 which was then stop pered. witl'rcap 40andcpla'ced within". container 362 Outer fronr about-950 Cl to-about105.0 C. and preferabl'y' about 100 so: as to resultin furnace tempera=tures withiir-windings26 and 28 ofi from about 75 C'..to:. about 100 C.higher thanthatof the central portion of the furnace: nace weremaintained at the: higher temperature to prevent dynamiclossbyvaporization and condensation of vapon'zable constituents.

The furnace was" maintained" at the temperatures and gradients indicatedin the paragraph preceding for: a.- period; of'about two hours afterwhich power 'to' terminals 31 and" 32 wasterminatedaand-switch 35was'closed so as to shunt' windingZS thuscreating a temperature gradientwith the high end of the gradient at the top of the furnace and the lowend of the: gradient at the bottomof the furtrace as themelt cooled.vUnder the conditions indicated the'temperature gradientwas from a highof about 1100 to. adow'ofzabout 900 C. This; gradient was main'tainedifor-aperiod.ofiahout one-hour. after which the currentfwasiturnedf oiiandi the melt permitted to return to.

AgSbse was preparedrin. accordance with the above outline. using amixtureof 2022 grams of. silver, 22.85 grams ot'antitnony. ancL29.6.-grams of selenium. These materials were thoroughly mixed with a spatulabefore beingplacedlin.crucib]e 37. The final ingotwassinglepliase,.liad'a.meltingpointlof610 C., an energy gap of about.0.6electronvol't, a resistivity. of about 0.002 ohmcentimetersandwasofIp-type. conductivity. The ingot was. zon'erretined by the passageof.Z0 zones resulting in. an.ii1crease inresistivity to. avalue. of about0.02 ohm.- centi'meters. The rectification. ratio. was about 2:1.

Example 2 AgBiSe was prepared as above using a starting charge of:13249. grams of. silver, 26.13 grams of bismuth, and 19..74'grams.ofiselenium. The final material. was single phase, oflameltingpointof.780 C. and evidenced n-typeconductivity.

Examplei AgAEsS'e was prepared as-above-using 20.22 grams 0t silver.14.05 grams of-arsenic' and 29.6 grams of selenium;

The fihal ingot was single phase, had a melting point of" 390 C. and an'.energy'=gap'of about 0.9 electron volt.

In all of the examples above; itwas' found that'particlesizefofistartingconstituents was-not critical. Actual .par'

tieless-iites'varied fromz ao'out"0.-1 to' about" 025 The upper. andlower' portions of the fur Each of the compounds of this invention.manifests either" hole or electron conductivity and is, therefore; anextrinsic semiconductor as made. That the conductivity type of thesecompounds is an extrinsic characteristic probably due to incorporationof significant impurities as in the well-known germanium and siliconsystems, is further evidenced by the resistivity gradient whichresultsupon zone-refiningand also by, changes in resistivity and inconductivity type' by doping. As is" indicated in Example 1 above,zone-refining of' silver antimony selenide by'passa'ge: of 20. zones.through the ingotas. prepared; above'resulted in a. tenfold increaseiniresistivitys.

The conductivity type of the compounds of this invention has beensuccessfully converted. by the. use, of small amounts of dopingelements. In accordance with conventional doping theory the conductivitytype of any one of the ternary compounds herein may be caused toapproach n-type material by substitution of any one of the elementsofthe compound by. any element having a larger number. of-Telectrons in"its'outerringand' may be caused to approach p-type by such substitutionwith one element" havingasmaller number of sueh.electrons. The determination. of. practicalsignificant impurities additionally dependsuponpliysical and chemical characteristics which will permit suchsubstitution without appreciably affect;- ing' the crystallography. andthe chemical. composition of the compound; A substantial amountofstud'y; lias been given these considerations in the field of'dopingofsemiconductive materials in general and criteria upon which an accurateprediction may be premised are avail'-' able in the. literature,.see forexample, L. Pincherl'e. and J. M. Radcliffe, Advancestin Physics,.volume5",.No. 19; July 1956, page 271. In general, ithasbeen foundthat if? theextrinsic element so chosen is. chemically compatible with both thecompound and the. atmosphere to which the compound is exposed: duringhigh temperature processing, this element, if it hasan atomic radiuswhichis fairly; close to that" off one of; the elements of the ternarycom.- pound, will'seek out a' vacancy" in the'lattice and will'occupy asite" corresponding with thatiot" that element? of the compound. Dopingmay be efiect'ed' also by" intro= duction of'srnall atomswhichappearto'occupy'interstitial positions as, for: example, lithium ingermanium and hydrogen" in zinc; oxide.

In accordance with the above, it has been found'that iodine from groupVII of. the periodic table having. a" radius ot*1'.33 A. willreadilyoccupy a. selenium" site; in any'one'of' the compounds'of thisinvention and thereby. act as a significant impurity inducingn-typeconductivity: Selenium isan elementifrom the sixth group of the;periorlictable and'hasaradiusof' 1.17 A. Other'el'ement's from theseventh group of the periodic table haveza'siinil'ar efiect. It hasbeen". found". that chlorine, for example, havinga' radius'of0.99'A.also substitutes for selenium and induces n=type" conductivity althoughit is'not gens erallyconsideredto b'ea desirable significant impuritysince it" is extremely reactive with moisture andprecaw' tionsmustbetaken to keep the atmosphere dry during its introduction: Startingwith a;compoundherein'whicli: exhibits p-typeconductivity'as made, p-njunctionshave been produced by difiusing iodine into the solid'materialiSuch p'-n junctions have exhibitedrectification'properties: Manganesehaving, an atom radius of 1.17 A; is also effective as a; donor.

Whereas the group VII elements act"assignificant'inr+ purities probably"by substitution. for selenium; which hypothesisis premised primarily onthe range of Z atomiir radii involved, conductivity type has also beenaffected? by use of elements which in accordance with" thepresenttheory-must, upon incorporation, occupy the site" ofi'tlie silver atom.For example, it has been foundthat'earl miumand-zincfrom groupIlandantimony from group-V" all have the efiect of inducing,netypeeondhctivity; in" AgSbSeg. Assumingtliatdopingproceedsbytltesubsti tutionmethods which is fairly'well establislied, in.

stance of the common semiconductor systems it is hypothesized that sinceeach of these materials produces n-type conductivity and must,therefore, introduce an excess of electrons, and since the only elementin the compound having fewer electrons is silver that, therefore,substitution must be for that element. That such substitution in facttakes place is further to be expected in view of the range of atomicradii involved, the atomic radius of cadmium, zinc, antimony and silverbeing in that order 1.41 A., 1.25 A., 1.41 A. and 1.34 A.

That the conductivity effect of the presence of an excess of antimony isdue not merely to the excess but rather to a substitution for the groupI element contained in the compound is premised primarily on theconclusions reached on crystal structure studies made on AgSbSe and onother semiconductor systems See, for example, L. Pincherle and I. M.Radclifie, Advances in Physics, volume 5, No. 19, July 1956, page 271.

In common with experience gained from studies conducted on othersemiconductor systems, it is found that addition of impurities inamounts of over about 1 percent by weight may result in degeneratebehavior. Amounts of significant impurity which may be tolerated aregenerally somewhat lower and are of the order of 0.01 atomic percent.However, it is not to be inferred from this observation thatsemiconductor devices of this invention must necessarily contain 99percent or more of a particular semiconductive compound disclosedherein. It is well established that desirable semiconductive propertiesmay be gained by the combination of two or more semiconductivematerials, for example, for the purpose of obtaining a particular energygap value. For this reason, any one of the compounds herein may bealloyed with any other such compound or with any other semiconductivematerial without departing from the scope of this invention.

This invention is limited to semiconductor systems utilizing one or moreof the compounds of the formula AgXSe where X is antimony, bismuth orarsenic and to devices utilizing such systems.

Although the invention has been described primarily in terms of specificdoping elements and specific devices, it is to be expected that thewealth of information gained through studies conducted on othersemiconductor systems may be used to advantage in conjunction with thisinvention. Refining and processing methods, as also diffusion andalloying procedures and other treatment known to those skilled in theart, may be used in the preparation of materials and devices utilizingthe compounds herein, without departing from the scope of thisinvention. Other device uses for the compounds herein are also known.

What is claimed is:

1. A semiconductor system containing a compound in accordance with thecomposition AgXSe in which X is 6 an element selected from the groupconsisting of Sb, Bi and As.

2. A semiconducting material consisting essentially of at least 99percent by weight of a compound of the composition AgXSe in which X isan element selected from the group consisting of Sb, Bi and As.

3. A semiconducting material in accordance with the composition of claim2 containing up to 0.01 atomic percent of a significant impurity.

4. A semiconducting material in accordance with claim 3 in which thesignificant impurity is an element of group VII of the periodic table inaccordance with Mendelyeev.

5. A semiconduccting material in accordance with claim 4 in which thesignificant impurity is iodine.

6. A semiconducting material in accordance with claim 4 in which thesignificant impurity is manganese.

7. A semiconducting material in accordance with claim 3 in which thesignificant impurity is an element of group II of the periodic table inaccordance with Mendelyeev.

8. A semiconducting material in accordance with claim 7 in which thesignificant impurity is zinc.

9. A semiconducting material in accordance with claim 7 in which thesignificant impurity is cadmium.

10. The semiconductor system of claim 1 in which 99 percent by weight ofother material therein contained exhibits semiconducting properties.

11. A semiconductor device consisting essentially of a body of materialof the system of claim 1 and having at least one rectifying contact madethereto.

12. The device of claim 11 in which rectification is by means of apoint-type electrode making contact with the said body.

13. The device of claim 11 in which the rectifying contact is made bymeans of a p-n junction.

14. A semiconductor transducing device comprising a body of material ofthe composition of the system of claim 1, said body containing at leastone p-n junction.

15. A semiconductor transducing device comprising a body of material ofthe composition of the system of claim 2, said body containing at leastone pn junction.

References Cited in the file of this patent UNITED STATES PATENTS2,762,857 Lindenblad Sept. 11, 1956 FOREIGN PATENTS 1,120,304 FranceApr. 16, 1956 OTHER REFERENCES Mellor: Comprehensive Treatise onInorganic and Theoretical Chemistry, Longmans, Green and Company,London, 1923, vol. 3, page 7.

Hackhs Chemical Dictionary, 3rd edition, pages 226 and 774.

11. A SEMICONDUCTOR DEVICE CONSISTING ESSENTIALLY OF A BODY OF MATERIALOF THE SYSTEM OF CLAIM 1 AND HAVING AT LEAST ONE RECTIFYING CONTACT MADETHERETO.